An air suspension is often used for realizing sophisticated control in connection with an electronic control device because it is comparatively easier to change the suspension characteristics.
Control items by such suspension include adjustment of body height, adjustment of vehicle attitude and can be realized by supplying the adequate amount of air to the air chamber of air suspension of each wheel and by exhausting the air therefrom. However, for the sophisticated suspension control, it is essential to adequately change supply or exhaust rate of air in accordance with respective control purposes. It is preferable for ride comfort to equalize a speed of a vehicle height increase to that of a vehicle height decrease.
A device for controlling the vehicle height has been proposed and developed in order to change the supply rate of air to the air suspension in accordance with these purposes. The reserve tanks are provided in the device to reserve the compressed air in the circuit and two systems of paths, a small diameter pipe and a large diameter pipe, are provided to connect such reserve tanks to the air chambers of suspension unit of respective wheels (Japanese Laid-open Patent Application No. 119623/1985).
Japanese Published Examined Patent Application No. 28589/1975 and Japanese Laid-open Patent Application No. 27711/1986 disclose an air supply system in which both high and low pressure reserve tanks are provided therein, and a compressor compresses air from the low pressure reserve tank and supplies the compressed air to the high reserve tank.
The system of the former prior art and the related various systems have been proposed in order to change a controlled rate of vehicle height during increasing and decreasing of the vehicle height. However, it is rarely taken into account that an increasing speed and a decreasing speed are equalized. Increase and decrease of the vehicle height are executed by supplying and discharging of a fluid to and from the fluid suspension. If the supply speed and the discharge speed are not different from each other and also the increase and decrease speeds of a vehicle height, for example, if air is supplied to either one of right and left suspensions and discharged from the other during turning, discomfort of riding results. In the system of the latter prior art, for example, both high and low pressure reserve tanks are provided, and the high pressure reserve tank supplies the fluid to the fluid suspension, and the discharged fluid from the fluid suspension is reserved into the low pressure reserve tank.
The volume of a fluid chamber should be variable for a vehicle height adjustment or a spring constant adjustment, for this reason, a resilient diaphragm is usually provided. In this diaphragm, it is difficult to maintain the pressure of the fluid chamber of the fluid suspension extremely high in view of pressure proof and durability. Consequently, the pressure of the fluid chamber is usually maintained in a comparatively low pressure. On the other hand, the pressure of the high pressure reserve tank is maintained in a substantially high pressure on account of a volume efficiency of a motor-car and the like. In general, the following equation is obtained: EQU Ph-Pa&gt;Pa-P1 (1)
where Pa represents the fluid chamber's pressure, Ph represents the high pressure reserve tank's pressure, Pl represents the low pressure reserve tank's pressure. In the prior fluid suspension circuit, the above pressure difference is not taken into consideration so that the speed difference between fluid supply and discharge is derived, and the ride comfort becomes worse.
Furthermore, for example, to simply decrease the vehicle height, the air exhausting speed from the respective air suspension of wheels should preferably not be very fast so as not to shock the vehicle. Conversely, during turning, it is preferable that the air exhausting speed is fast so as to stabilize the vehicle attitude, also a driver occasionally may wish to vary the vehicle adjusting speed.
Nevertheless, in the prior art, the fluid in the fluid chamber is exhausted through a predetermined line by opening a predetermined valve so as to decrease the vehicle height or a spring constant. For example, when the air is used as fluid, the exhaust air is directly discharged from the air chamber of the suspension, or is accumulated in the low pressure reserve tank as disclosed in Japan Laid-open Patent Application No. 27711/1986.
However, the above-mentioned method makes a suspension configuration constructed by such as a pipe, a valve quite complicated and the vehicle weight is increased because it is necessary to prepare two lines, respectively, for fluid supply and discharge lines to vary fluid supply and discharge speeds.